1. Field of the Invention
The invention relates to solar arrays of silicon particles cast in glass or transparent plastic that generate electricity.
2. Description of the Prior Art
Systems for producing useful energy by conversion of the rays of the sun to electricity are well known and constantly being developed and improved due to the promising economics of the sun being the primary source of energy involved. Traditionally, these systems have been based upon silicon slices cut from high-purity single crystal boules. The slices are mechanically and chemically polished, diffused to form large area photodiodes, and interconnected as photovoltaic (PV) arrays. Arrays of this kind, however, are expensive. This is because single crystal silicon boules are themselves expensive, kerf loss and polishing wastes 30%-50% of each boule, and a large amount of silicon is required per unit area of array because the slices must be sufficiently thick to survive handling during processing. As a result, the cost per unit kilowatt hour of electrical power output generally exceeds the cost of power purchased from electrical utilities, and the market for such arrays is thus limited to special applications where utility power is not an alternative.
One approach to solving the cost/performance problem is to prepare solar arrays composed of a thin transparent matrix such as glass or plastic embedded with nearly spherical silicon particles, each slightly larger than the matrix thickness and each having regions of both P-type and N-type conductivity. All regions of one conductivity type are electrically connected together (e.g., with metallization on the backside of the array) while all regions of the other conductivity type are independently electrically connected together (e.g., with metallization on the frontside of the array). When irradiated with light, the particles act as an array of discrete photodiodes connected in parallel, and a photovoltage of between 0.45 V and 0.60 V develops between the two common interparticle connections. Several embodiments of array structures based on silicon particles are found in the following U.S. Pat. Nos.: 2,904,613 of M. E. Paradise, 3,025,335 of E. L. Ralph, 3,040,416 of Matlow et al., and 3,998,659 of G. F. Wakefield. Cost/performance of these arrays is potentially superior to that of arrays prepared from single crystal silicon slices since, while both have roughly equivalent theoretical solar-to-electricity conversion efficiency, the silicon particle arrays will be considerably less expensive. This is because the techniques for preparing particles are inherently inexpensive and generate little or no waste. Moreover the three-dimensional nature of the particles and their small size ensures that much less silicon is required per unit area of array to achieve comparable active P-N junction area.
In practice, however, the potential advantages of silicon particle arrays have not been realized in that actual solar-to-electricity conversion efficiencies have typically not exceeded a few percent. The following three factors listed in descending order of importance, have been mainly responsible for limiting efficiency:
shorted particles internally shunt the photocurrent. PA1 series resistance imposed by the metallizations used for inter-particle connections degrades the fill factor. PA1 poor light-gathering characteristics of the array structure degrades the photocurrent.
The seriousness of shorts can be readily understood by means of an example. Assume a 1 cm.sup.2 silicon particle array is composed of 500 0.04 cm dia. particles each with a core resistivity of 1.0 .OMEGA.-cm and each of which generates an operating current of 50.times.10.sup.-6 amp in bright sunlight. The array operating current would be 25.times.10.sup.-3 amp. A shorted particle will represent a shunt resistance of about 34 .OMEGA. and its presence in the array will shunt away about 14.7.times.10.sup.-3 amp at an operating voltage of 500 mV. This represents an efficiency reduction of 59%.
Kilby et al. in U.S. Pat. Nos. 4,021,323 and 4,100,051 describe a technique for using silicon particle arrays in such a way that the problem of shorts is avoided. The arrays contain both P-type particles with N-type skins and N-type particles with P-type skins embedded in a transparent glass matrix. On the backside of the matrix the core conductivity regions of the particles protrude and are interconnected with appropriate electrically conductive metallization. The skin conductivity regions of the particles protrude from the frontside of the matrix where they are individually coated with films of catalyst metal that are, in turn, in contact with an electrolyte solution. Due to the potential difference set up between the two types of silicon particles under sunlight, the solution is electrolyzed. The reaction products are stored and later recombined in a fuel cell to generate electricity. During electrolysis, each particle acts independently. Shorted particles do not reduce the output of other particles, they simply do not generate reaction products themselves. Although this technique deactivates shorts, it does not provide an optimum solution to the problem. This is because solar-to-electricity efficiency is reduced by the energy losses involved with electrolysis, both N/P and P/N particles are required, more costly corrosion-resistant materials must be used, and encapsulation of the electrolyte above the array surface presents engineering problems. However, this technique may be attractive if the electrolysis can be combined with a requirement for electrical energy storage.
A way to minimize the effect of shorts in silicon particle arrays has been suggested. An aluminum foil is simultaneously thermocompression bonded to one conductivity region of each particle in an array such that a fixed contact resistance in series with each particle is effected. If the incidence of shorted particles is low, by controlling the contact resistance, their effect can be reduced to a minimum with only a slight sacrifice of output power. For example, placing a 400 .OMEGA. resistor in series with each particle in a 1 cm.sup.2 array comprising 500 particles each generating 50.times.10.sup.-6 amp at an operating voltage of 500 mV, would reduce output power by 4% due to I.sup.2 R losses in the 400 .OMEGA. resistors and result in only an additional 5% power loss if a shorted particle were present. This technique, however, also does not provide an optimum solution to the problem. Formation of interparticle connections by thermocompression bonding of aluminum foil is not compatible with glass or plastic matrices and appears to be practical only if aluminum foil is used for both connections. Achieving and maintaining the desired contact resistance requires exacting control over the bonding parameters. Moreover, the undesirable I.sup.2 R loss is unavoidable, and the technique fails to provide adequate protection against shorted particles if their concentration exceeds about 1/2%.
While silicon particle solar arrays of the type described above and elsewhere in the art may operate satisfactorily, it is desirable to increase their solar-to-electricity conversion efficiency in order to provide a maximum of electrical energy from a photovoltaic solar array of predetermined dimensions.
Therefore, it is an object of this invention to provide a solar array, comprising silicon particles embedded in a transparent matrix of insulator material, in which shorted particles are isolated by such means that efficiency is increased to the maximum possible extent.
It is an additional object of this invention to provide a solar array consisting of silicon particles embedded in a transparent matrix of insulator material, which is provided with a transparent interparticle connector film such as tin or indium oxide whose conductivity has been increased by the inclusion of metal whiskers.
It is yet a further object of this invention to provide a solar array comprising silicon particles embedded in a transparent matrix of insulator material, in which the surface of the matrix in contact with interparticle connector films is microscopically smooth.
It is a further object of this invention to provide a solar array comprising silicon particles embedded in a transparent matrix of insulator material, in which a light-reflective surface is provided to direct rays of light that have penetrated the matrix between particles back onto the sides of the particles where they can be usefully absorbed.
It is still a further object of this invention to provide a solar array of silicon particles embedded in a transparent matrix of insulator material in which the particles are arranged in a two-dimensional hexagonal close packed arrangement.